The present invention relates to an apparatus and method for driving a gas discharge lamp, and in particular for dimmably or non-dimmably driving fluorescent lamps or tubes.
Fluorescent lamps or tubes are widely used in the home, office and in industry to provide lighting. Such lamps generally consist of a tubular glass envelope, up to 2.44 m (8 feet) long, filled with an inert gas such as krypton or argon which when electrically excited in a gas discharge irradiates a fluorescent coating, such as a powder comprising a (Tb,Ce,Gd,Mg) borate, a (Eu,Ba,Mg) aluminate and a (Y,Eu) oxide, on the inside of the glass. An example of such a tube, 1.22 m (4 feet) long, is the model xe2x80x98TLxe2x80x99D 36 Watt sold under the trade names xe2x80x9cSuper 80 (/840) New Generationxe2x80x9d and xe2x80x9cStandard (/33)xe2x80x9d by Philips Electronic and Associated industries Limited.
All gas discharge lamps, including fluorescent lamps, require a ballast to operate. The ballast provides a high initial voltage to initiate the discharge, then rapidly limits the lamp current to safely sustain the discharge. Ballasts are manufactured for three main classes of fluorescent lamp: preheat, rapid start and instant start.
Preheat operation lamp electrodes are heated prior to initiating the discharge. A starter switch closes, permitting a current to flow through each electrode. The starter switch rapidly cools down, opening the switch, and triggering the supply voltage across the arc tube, initiating the discharge. No auxiliary power is applied across the electrodes during operation.
Rapid start operation lamp electrodes are heated prior to and during operation. A transformer has two special secondary winding to provide the proper low voltage to heat the electrodes.
Instant start operation lamp electrodes are not heated prior to operation. Ballasts for instant start lamps are designed to provide a relatively high starting voltage, as compared with preheat and rapid start lamps, to initiate the discharge across the unheated electrodes.
Prior art document U.S. Pat. No. 4,464,606 discloses a circuit for dimmably controlling a pair of fluorescent lamps, in which a push-pull transistor pair is pulse width modulated to vary the duty cycle of a pulsed current supply to the primary of a transformer to the lamps.
It is desirable to be able to dim fluorescent tubes in order achieve increased energy efficiency when full, lighting is not needed. It is known that such tubes up to 1.83 m (6 feet) long can be dimmed with appropriate control circuitry. For example, the above-mentioned 1.22 m fluorescent tube may be dimmably controlled with high frequency regulating ballast sold by Philips Lighting Limited as model number BPL136R.
With reference to Philips Lighting data sheet PL 3322, such known ballasts suffer from a number of limitations. First, it is only possible to achieve adequate control over the dimmable light output for fluorescent tubes up to 1.83 m (6 feet) in length. Secondly, it is only possible to dim down to about 10% of full light output before the tube flickers out. Thirdly, the lighting efficiency of such dimming ballasts drops steadily as the light output falls, the efficiency being 56% at 25% light output and 27% at 10% output, as a result of increased thermal losses in the tube and ballast circuitry. Thus, the benefit of decreased electricity consumption is not fully realised at low power levels.
The reason for these limitations in performance appears to stem from the way conventional non-dimmable high frequency (hf) ballasts have been adapted for use as dimmable ballasts. A conventional hf ballast generates a pulsed voltage, typically at either 28 kHz or 35 kHz, modulated on and off at a low frequency (50 Hz or 100 Hz), with an on/off ratio of 50% so that there is no hf signal during each half-cycle. A conventional dimmable hf ballast reduces the on/off ratio so that the hf pulsed voltage becomes progressively less than 50% of the duty cycle. The hf pulses are therefore applied to the fluorescent tube for a lower average duty cycle and as fewer hf pulses are applied to the tube, the tube dims.
In general, a number of limitations have been noted with such dimmable systems. First of all, because conventional fluorescent ballasts include a choke with a substantial inductance, proportionately greater amounts of energy are lost in ohmic heating of the choke as the tube is dimmed. Secondly, as the tube is dimmed, a point is reached where the tube fails to strike properly owing to the increasingly large proportion of time when the hf voltage is not applied to the tube. The tube therefore tends suddenly to flicker off before it has been fully dimmed, owing to the increasingly discontinuous nature of the pulse train applied to the tube. These problems become worse for increased length of fluorescent tube and consequently it is believed that there are no commercially available dimmable or non-dimmable ballasts for 2.44 m tubes, and the dimmable ballasts available for 1.83 m tubes do not work as well as those for 1.22 m tubes. See, for example, the comprehensive online database to be found on the internet at http://light-light.com/ which lists all commercially available fluorescent lamps and ballasts. This database lists no commercially available dimmable or non-dimmable ballasts for fluorescent tubes longer than 1.83 m.
The fact that 2.44 m non-dimmable hf fluorescent tube ballasts are not commercially available is surprising, since there has been a trend since at least 1981 to use non-dimmable hf ballasts for improved energy efficiency whenever possible. High frequency ballasts are, however, known to suffer from various problems.
One problem results from the relatively greater power and hence current and voltage requirements of 2.44 m fluorescent tubes as compared with shorter tubes. Inefficiencies in the ballast circuitry, including transformers, result in excess heating within the ballast unit, which can be damaging to solid state circuit elements. The space within a typical fluorescent tube fitting is quite limited, and it is believed that the build up of heat owing to the relatively greater power requirements has meant that it has not been possible or economic to manufacture a high frequency ballast for a 2.44 m fluorescent tube with a commercially acceptable failure rate, e.g. of less than IA in the first year after installation.
Another problem is that the circuitry conventionally used generates what are known as xe2x80x9charmonicsxe2x80x9d and to transmit these harmonics back into the power supply grid. This is a particular problem in certain industrial situations where, for example, a factory may have many hundreds of 2.44 m tubes on a number of lighting circuits supplied through a local step down transformer. In such a situation, harmonics can lead to overloading of transformers, adding of current to the neutral in three phase electrical distribution systems, current/voltage surges or spikes due to circuit resonances with one or more of the harmonic frequencies, and interference with other electronic equipment.
As a result, standards have been introduced to limit the amount of harmonic distortion produced by high frequency ballasts.
It is an object of the invention to provide a circuit for a high frequency ballast for a gas discharge lamp that addresses these problems and which may be dimmable, and which may be used with certain types of gas discharge lamp such as high output 2.44 m fluorescent lamps which to date have not benefited from the increased efficiencies possible with high frequency operation.
According to the invention, there is provided an electronic circuit for controlling a gas discharge lamp, comprising generation means for generating a high frequency pulse train that may be applied to the electrodes of the lamp to light the lamp, means for connecting the means for generating a high frequency pulse train to an electrical power source, a choke to limit the current drawn by the lamp, characterized in that the circuit comprises means for producing a first series of pulses and means for producing a second series of pulses independently from the first series of pulses, and means for combining additively the first and second series of pulses to produce the high frequency pulse train.
In a preferred embodiment of the invention, the circuit is for a fluorescent lamp.
The term high frequency is intended to exclude frequencies above those used for mains supply, i.e. above 50 to 60 Hz. The value of the high frequency may depend on a number of factors, in particular the type of lamp and the physical size and power rating of the lamp.
The arrangement is such that the rms power level of the high frequency pulse train is determined by the first and second series of pulses, and in particular because the series of pulses are independent of each other may be set.
The use of two independent pulse trains combined additively also makes it possible for the voltages of the first and second series of pulses to be less than that supplied as the combined high frequency pulse train applied to the lamp. For example if the voltages of the two series of pulses are the same, then these can then be made to add together so that the combined pulse train has a voltage double that of the each of the series of pulses. The use of lower voltages improves safety and simplifies the design of the generation means.
The choke serves in use to limit the current drawn by the lamp once the gas discharge is struck, and also to provide a high voltage boost to initiate the discharge when the lamp is first started.
Preferably, the means for combining the first and second series of pulses includes the choke which connects together the first and second series of the pulses.
The means for combining the first and second series of pulses includes an isolating transformer means to electrically isolate the lamp from the power source. The output of the circuit would then be floating. It has been found that this helps to prevent capacitative transfer of high frequency voltage to the glass envelope of the lamp, which can cause a unpleasant sensation when the lamp is touched when it is on.
When the circuit is for controlling the light output of a gas discharge lamp, the circuit additionally comprises means for shifting the phase of the first series of pulses relative to the second series of pulses, the means for combining the first and second series of pulses thereby varying the width of pulses in the pulse train.
By varying the width of the pulses in the pulse train, it is possible to control the rms power supplied to the lamp.
For example, the circuit may comprise means to detect a variation in a supply voltage from the power source. The means for shifting the phase of the first series of pulses relative to the second series of pulses may then responding to a variation in the supply voltage so that the lamp output may be held steady as the supply voltage varies.
The lamp may then also be controlled dimmably, if the circuit comprises light level control means for setting a desired intensity of light output from the lamp. The means for shifting the phase of the first series of pulses relative to the second series of pulses may then responding to the light level control means so that the lamp output may be set at a desired level as the width of the pulses is varied.
It is also possible that the circuit can control a lamp according to the whether or not there is a need foe the light to be on. For example, the circuit may comprise motion detection means to detect motion of an object, such as a person, in the vicinity of the circuit. The light level control means may then respond to the motion detection means so that the lamp output may be set at a desired level according to the detected motion as the width of the pulses is varied.
Whether the circuit is for dimmably controlling or for steadily driving the lamp, the means for combining the first and second series of pulses preferably comprises a first transformer and a second transformer, the primaries of each transformer receiving respectively the first and second series of pulses, each of the secondaries having a tap which may be electrically connected to the contacts of the lamp and each having another tap electrically connected to a choke so that the choke combines the secondaries and the choke in series between the contacts. The choke is thereby in series with the pulse train.
The choke serves in use to give a high voltage boost if the lamp starts to flicker off at very low power levels, so ensuring that the circuit may control the lamp power close to zero without the need for complicated feedback and lamp drive control circuitry.
The choke also serves to round off any square edges on the high frequency pulse train as the lamp is striking, and it is believed that this effect is important at helping the lamp to work steadily at low power levels, and also to come on at low power levels without the need for any heater element pre-heating delay.
In a preferred embodiment of the invention, the circuit has paired outputs each pair of which provides a steady low voltage output which may be applied to heated electrodes of the lamp.
Then at least one of the transformers may have a secondary winding with a pair of taps that may be electrically connected to heater elements of the lamp. One of the secondary taps for the heater element may then be electrically connected to one of the secondary taps for the lamp contacts so that the heater elements can then receive high frequency pulses with a power level sufficient to heat the heater elements.
Preferably, this power level should be substantially constant and, in the case of the circuit for dimmably controlling the lamp, unaffected by the phase shifting of the first and second series of pulses with respect to one another.
The modulation means may vary the width of each pulse in the pulse train similarly, that is, so that the ratio of on/off time for each combined high frequency pulse is substantially the same.
It would, however, alternatively be possible to vary the width of each combined high frequency pulse in the pulse train dissimilarly, that is, so that the ratio of on/off time for at least some of the adjacent pulses in the pulse train are not substantially the same, so long as the gaps between pulses do not become so long that the pulse train becomes substantially discontinuous, so causing the tube to flicker off at lower average duty cycles.
The pulse train may comprise pulses of just one polarity, but preferably comprises pulses of both positive and negative polarity.
Circuitry such as that described above is not bulky and may readily be incorporated in a light fitting having contacts for a gas discharge lamp. Alternatively, the circuit may be separate from the light fitting, although it would be necessary to provide appropriate transmission lines, e.g. coaxial cable, and shielding to prevent stray leakage of electromagnetic radiation.
The invention will be further described by way of example to the accompanying drawings.